Our recent experience is that though the buses run fairly regularly, they are not super-consistant.
We were staying at POFQ wanting to go to AK for morning EMHours, starting at 8 AM. We got to the bus stop at 7:25 AM and there were already a few people waiting. The AK bus did not show up until almost 7:50 and there were more people waiting than could fit on a single bus by that time. During the time between when we got there and when our bus arrived, there were two MK buses, two Epcot buses, two MGM buses, and even a DTD bus, none of which opened until 9 AM. I was fuming that we didn't get to the park until 8:30 AM for the EMH.
However, other than that, the typical wait for the buses was between 5 and 15 minutes most days, and likewise coming back from the parks we never waited more than 15 minutes for a bus, and usually 5 to 10. Of course, we tried to avoid closing time.
SkierPete